Figure 6.

Artificial slowing down of action potential (AP) onset impairs encoding of high frequencies and decreases the speed of population firing rate response. (A1): Responses of a neocortical neuron to injection of the same fluctuating current (bottom trace, black) in control conditions (red) and in reduced extracellular [Na⁺] (blue). Recordings are from the soma. (A2) AP waveforms, phase-plots, and zoom-in of initial portion of APs in phase plots illustrate slower onset of APs in low extracellular [Na⁺] relative to control. (B) Frequency response functions of the neuron from A recorded in control conditions (red) and then in low [Na⁺] (blue). Transfer functions were cut at intersection with respective 95th percentile of N = 500 transfer functions obtained with shuffled AP timings (gray; see Ilin and others 2013 for details). (C) Averaged frequency response functions measured in control conditions (red; N = 18 neurons) and in low [Na⁺] (blue; N = 7). Transfer functions were cut at respective 95th percentiles (pale color lines). The bar below the plot shows high frequency range in which encoding in low [Na⁺] experiments is significantly impaired (**P < .01) relative to control. (D) Changes of population firing rate in response to the onset and offset of small current steps immersed in fluctuating current (bottom), in control conditions (red) and in the medium with reduced extracellular [Na⁺], which leads to slower AP onset dynamics (blue). Each peristimulus time histogram (PSTH) was calculated using N = 9100 responses. Vertical bars above the onset and offset of steps in histograms show averaged AP duration. (E) Probability of step detection vs. time after step onset, for populations of N = 100, 500, or 3000 neurons. Note that for samesize populations detection time in low [Na⁺] is about twice as long as in control. Modified from Ilin and others (2013).